using System;
using System.Linq;

class Program
{
    static void Main()
    {
        int n = int.Parse(Console.ReadLine());
        int[] x = new int[n];
        int[] y = new int[n];
        for(int i=0;i< n; i++)
        {
            string[] s = Console.ReadLine().Split();
            x[i] = int.Parse(s[0]);
            y[i] = int.Parse(s[1]);
        }

        int ans = y[0] - x[0];

        for(int i=0;i< n; i++)
        {
            if (y[i] - x[i] != ans)
            {
                ans = -1;
                break;
            }
        }

        if (ans <= 0) ans = -1;

        Console.WriteLine(ans);
    }
}